Phenol formaldehyde resins are known. These resins are typically made by condensing phenol with formaldehyde in the presence of a basic or acidic catalyst. The product resin comprises a crosslinked network of phenyl rings connected by methylene bridges. The molecular weight and degree of crosslinking of the resin may be increased by increasing the amount of formaldehyde. Molecular weight and degree of crosslinking can influence physical properties such as heat and flame resistance and mouldability. However, there is a practical limitation to the amount of formaldehyde which can be used. At a formaldehyde phenol ratio of above about 2:1, the rate of the crosslinking reaction is difficult to control and is considered too reactive for commercial resin production. For this reason, the maximum ratio of formaldehyde in conventional resins is about 1.5:1.
In an earlier patent application No. WO 92/1758 a phenol formaldehyde resin having a relatively high formaldehyde ratio is described. This resin is prepared by mixing two separate phenol formaldehyde mixtures. A first resole is prepared by mixing phenol and excess formaldehyde in the presence of a basic catalyst. After a period of time, a stabiliser comprising glycerol and a dicarboxylic acid is added to control further reaction. Water is then removed under vacuum and methanol as solvent is added. Water is generally present in the reactants and is also produced by the condensation reaction between phenol and formaldehyde. The second resole is prepared by mixing phenol and formaldehyde in the presence of an acidic catalyst. In this case, only a marginal excess of formaldehyde is used. After the phenol and formaldehyde have reacted, water is removed under vacuum and a solution comprising zinc chloride, an alpha hydroxy acid, p-toluene sulphonic acid and sulphuric acid in a non-aqueous solvent is then added. To produce the product resin, an excess of the first mixture is mixed with the second mixture and the final mixture is allowed to cure.
The above process is essentially a two pack method for preparing a phenol formaldehyde resin. This method requires the preparation of two different types of resole, each resole being prepared under different catalytic conditions. The method also requires separate storage of each component until use.
It is an object of the present invention to provide a one step method for the preparation of a phenol formaldehyde resin having formaldehyde in excess or to provide the public with a useful choice.